The present invention pertains to rock cutter assemblies, and more specifically to rock cutter assemblies for use on raise boring apparatus wherein a plurality of independently rotatable cutting units are employed on the raise boring head.
U.S. Pat. Nos. 4,274,496 and 4,399,879 issued to Liljekvist disclose a cutter assembly with cutting means 53 in the form of hard metal inserts which are fitted in bores in the body of the cutter. The inserts 53 are positioned in rows 45-48 which extend circumferentially around the cutter. Multiple rows of cutting means 53 are required on each cutter because of the limited space near the center of any earth boring apparatus (raise reamer, tunnel borer, etc. ). FIG. 6 shows that each row of inserts 53 increases in diameter. The conical shape of the cutter is to limit the amount of skidding when the cutter rolls along the rock face. To totally eliminate the skidding would require changing the cone angle for each pair of cutters 13 and 14 so that the true rolling radius passes through the center of the reamer, which is not practical. If the cutter is located inside or outside of the true rolling radius, the cutting means 53 will skid. The skidding action significantly reduces the wear life of the cutting means. Also, the greater the distance between rows 45 and 48 the greater the skidding.
It is desirable to employ cutting units on a raise boring head with a minimum distance between rows, with a single row or disc being optimum. However, due to the small surface area of the raise boring head, there is not enough room thereon to place the requisite number of rotatable cutting units if each cutter assembly employs only a single disc.
U.S. Pat. No. 4,298,080, issued to Hignett discloses a rock cutter comprised of a pair of disc cutters 1 disposed in parallel and independently rotatable about tubular shaft support 2, the latter in turn being supported by a mounting pedestal 3. According to Hignett, the tool can be attached to the head of a tunnel boring machine, raise borer, or the like by fasteners 4. However, the cutter in Hignett employs thrust bearings to take the radial and thrust loads which occur during rock boring. The thrust loads induced into the thrust bearing are reacted by the end of the tapered rollers sliding against the guide flange on the bearing cone. This sliding action creates frictional heat which is detrimental to the life of the lubricant. The sliding action is particularly negative in rock cutter assemblies because the bearings rotate very slowly and are subjected to excessive impact loads. Both the high loading and slow rotation allows the lubrication film between the end of the tapered rollers and the guide flange to dissipate or break down. The lack of lubrication increases the amount of heat created by the sliding action. More importantly, in a rock cutting application, if muck contaminates the bearing cavity, the contaminates will prevent the sliding action which will invariably lock up the bearings.
Similarly, Atlas Copco has manufactured a cutter assembly for use in tunnel boring having a plurality of rotatable cutting units thereon which, as in Hignett, employ tapered roller bearings or thrust bearings, as disclosed in Atlas Copco Design No. 5107600040. An additional disadvantage with the use of tapered roller bearings is that, while the overall length of the cutter assembly is less than a plurality of cutter assemblies having individual cutting units, the length of the Atlas Copco and Hignett cutter assemblies are still excessive due to the use of tapered roller bearings.
U.S. Pat. No. 3,358,782 issued to Bechem, discloses a cutter assembly for use in bore hole enlargement. As shown in FIG. 1 of Bechem, the cutter assembly includes a spindle 12 with a plurality of cutting ribs 10, 14 and 17 thereon, which rotate independently around spindle 12. Unlike the above cutters where the two or more cutting units are rotatable on separate bearings, the cutting ribs of Bechem are configured such that each cutting rib rotates on a sleeve portion of the neighboring cutting rib with the cutting ribs having increasing diameters.
U.S. Pat. Nos. 4,815,543 and 4,736,987, both issued to Lenzen et al., disclose rock cutting assemblies having a drive shaft 10 and two rollers 12 and 14, the axes of which are displaced with respect to each other. Rollers 12 and 14 include hub portions 16 and 18, respectively, which are integral with shaft 10, and peripheral annuli (cutters) 20 and 22 which are rotatably mounted on the hubs 16 and 18 on suitable bearings 24. It is important to note that the axes of the two rollers in Lenzen et al. are displaced and are not colinear.
A need thus exists for a raise boring cutter assembly having a plurality of independently rotatable cutter units thereon such that a sufficient number of cutter units can be loaded onto the raise bore head.
A need also exists for the above type of cutter assembly wherein the overall length of the cutter assembly is minimized in order to maximize the number of cutter units that can be configured on the raise bore head.
A need also exists for the above type of cutter assembly in which the bearing type and configuration is maximized to decrease the likelihood of lock-up and minimize the generation of heat due to friction.
A need further exists for the above type of cutter wherein bearing life can be extended by providing a rotatable bearing surface.